Is 20 mph speed limit practical?

A Bill was presented to the Scottish Parliament last week by Green MSP Mark Ruskell, seeking to introduce a law making 20 mph the default speed limit in built-up areas across Scotland.

The bill was introduced with the support of 12 members from the ruling SNP, seven from Scottish Labour, five from the Scottish Green Party and one from the Scottish Liberal Democrats.

There already are many 20 mph zones across Scotland’s towns and cities, but the bill is aimed at reducing many current 30 mph speed limits to 20 mph. Under the proposed bill, Scottish local authorities would still be able to designate 30 mph as the limit on certain roads. But, the default urban limit would be 20 mph and they would have to follow a procedure – similar to that currently required to lower 30 limits to 20 mph – to raise the limit on any road to 30 mph.

Road safety charity Brake supports the speed limit reduction.

“At slower speeds, driver have more time to identify potential hazards and take action to avoid them,” say the charity, adding that at 20 mph the average stopping distance is 12 metres, rising to 23 metres at 30 mph according to the Highway Code.

It is also an undoubted fact that, if the worst happens, accident injuries are reduced dramatically at lower speeds.

But, just how practical and sensible are 20 mph speed limits? They make obvious sense in residential streets and busy city centres, but this bill seems to be aimed at extending 20 mph to the majority of roads in our towns and cities.

In these environmentally-aware times do we know how 20 mph speed limits would impact on fuel usage? At 30 mph a car may be travelling at very low revs in fourth, or even fifth gear. At 20 mph, it is likely that drivers will have to change down to fourth, or third gear, resulting in the engine running at higher revs and burning more fuel.

As has already been shown with speed bumps, slow-moving traffic produces significantly more localised pollution. A 20 mph speed limit would do the same, again with drivers using lower gears and higher revs.

It is ironic that when trying to clear traffic jams police officers are often to be seen exhorting drivers to speed up. The reason is, quite simply, that faster traffic clears congestion quicker than slow movement. Do we really know and understand what would happen on our commute if traffic was slowed to 20 mph?

Frustration causes accidents. We know that. It is emblazoned on warning signs on our roads and motorways. Would extensive 20 mph speed limits increase frustration and, thereby, increase the number of accidents? We really need to know, before we blindly introduce more extensive 20 mph limits.

What about driver concentration? It seems more likely we will let our minds wander when driving at low speeds. Some may even be tempted to fiddle with mobile phones, radios and the like.

What studies have been done into the economic issues of a 33% reduction in traffic speeds? What will be the impact on delivery deadlines and lost work time? Again, these need to be fully understood.

Finally, the big question. How enforceable is a 20 mph speed limit? The 30 mph limit has a reasonable acceptance and, therefore, reasonable compliance. Will drivers see 20 mph as a sensible speed and, thereby, will they generally accept and comply with it?

The argument of road safety is an easy win for those who want us to reduce our speeds. It is impossible to argue that slower speeds will reduce the severity of any accidents that happen – especially the most horrifying accidents that involve pedestrians.

But, we need to stop and think about the repercussions of a rush to reduce speeds.

After all, Britain’s roads are currently some of the safest in the world. We should, therefore, be especially careful that we don’t do something that has unintended consequences.